The progression of a cancer from the initiating cancer stem cell (CSC) to clinical significance, and ultimately to recurrence after treatment failure, is the culmination of a continuously evolving reciprocal interaction between the CSC and the stem cell niche. The hypothesis of this proposal is that androgen deprivation perturbs the critical role of the prostate endothelial cell in regulation of the androgenic milieu of the prostate tissue microenvironment, and in regulation of the stem cell niche, facilitating/accelerating the emergence of castration-recurrent prostate cancer To validate the hypothesis, the following Specific Aims are proposed. Aim 1 will characterize the role of endothelial cell caveolae in transcytosis of circulating androgens across the endothelial cell barrier, in endocytosis of circulating androgens resulting in AR-transactivation of genes associated with endothelial cell homeostasis and signaling, and in non-genomic signaling mediated through cell surface receptors for circulating androgens. Aim 2 will determine if androgen-deprivation induced killing of prostate endothelial cells results in transient, or irreversible, perturbation of the endothelial cells of the prostate microvasculature resulting in dysregulation of the tissue androgenic milieu (characterized using LC/MS/MS) and in creation/unmasking of unique, targetable epitopes or vascular addresses (characterized using phage peptide-display technology). Aim 3 will identify the effect of androgen deprivation on genes and gene pathways that are differentially expressed in prostate CSCs compared to adult stem cells (ASCs), identify the cell surface epitope fingerprint of prostate ASCs/CSCs, and determine if perturbation of the stem cell niche results in epigenetically-modulated dysregulation of the HNF-4(square) transcription network in prostate CSCs. The data and reagents generated in Aims 1-3 will support the studies of Aim 4 focused on development of novel approaches for utilizing androgen-deprivation to enable prostate-specific therapies by validating that: androgen deprivation exposes the ASC/CSC to targeted therapy; transcytosis of circulating androgens can be inhibited without perturbation of endothelial cell homeostasis; and endothelial cell initiated signaling to ASCs/CSCs can be reprogrammed to induce the stem cell to exit the niche and commit to differentiation.